CTS Guide- Chemical Reactions pp 152-153- Section IV Research Summaries
Chemical Change
Some students think when water boils, it breaks down into atoms of hydrogen and oxygen (Mayer 2011).
Middle and high school students’ ideas about chemical change tend to be dominated by the obvious features of the change (AAAS 2009).
Students experience difficulty in discriminating consistently between a chemical change and a physical change. Evidence for this comes from a number of studies. For example, Ahtee and Varjola (1998) explored 13 to 20-year olds ideas about what kinds of things would indicate a chemical reaction had occurred. They found that about 20% of the 13–14-year-olds and 17–18-year-olds thought dissolving and change of state were chemical reactions. Only 14% of the 137 19–20-year-old university students in the study could explain what actually happened in a chemical reaction.
Although in science the term chemical change refers to processes in which the reacting chemical substances transform into new substances, several studies have found that students often use the term chemical change to encompass a wide variety of changes including physical transformations, especially when the color of a substance changes. How well students make a distinction between chemical and physical changes may depend on their conception of substance. In general, students have difficulty developing the idea of chemical combination of elements until they are able to interpret what combination means at a molecular level (Driver et al. 1994).
In Abraham, Williamson, and Westbrook’s study (1994), students confused chemical and physical changes. There were indications that they had memorized the terminology rather than developed conceptual understanding.
A study by Abraham et al. (1992) presented eighth grade students with a chemical change in which a glass rod is held in the flame of a burning candle and a black film forms on the rod. To show understanding of chemical change, students were expected to identify the transformation that took place and know a new substance was formed, not just a different form of the same substance. Fifteen percent of the students questioned showed some understanding of chemical change. Fifteen percent had some understanding of chemical change but then provided evidence of a physical change, and some said the change was not a chemical change because no chemicals were involved. Seventy percent of the students showed no understanding that a chemical change had occurred with the burning of the candle and formation of the black film on the glass rod.
In a study of 100 high school students who completed a unit on chemical change, most students failed to use atoms and molecules in their explanations even though they had been emphasized in their chemistry course. Some students also listed heat, cold, or decay as reactants or products. Although most students in the classes could state the definition of a chemical reaction and balance chemical equations, they still had difficulty describing chemical changes (Hesse and Anderson 1992).
Andersson (1991) investigated children’s notions of chemical change and found they appear to fall into six categories: (1) it just happens; (2) matter just disappears; (3) the product materials must have been inside the starting materials; (4) the product material is just a modified form of the starting material; (5) the starting material just turns into the product material; and (6) the starting materials interact to form the product materials.
Stavridou and Solomonidou (1989) explored ideas held by Greek students ages 8 to 17 by presenting them with 18 different phenomena to classify as a chemical or physical change. They found that students who used the reversibility criterion were better able to distinguish between chemical and physical changes than students who did not consider reversibility. The students who used the reversibility criterion considered chemical changes to be irreversible, which could pose a problem in understanding chemical reactions. Both groups used criteria that were macroscopic in character.
Vogelezang (1987) found that students who regard ice as a different substance from water are likely to consider freezing water or melting ice as a chemical change.
Briggs and Holding (1986) found that 75% of the secondary students they sampled thought a change in mass was evidence for a chemical change.
Rusting
Students have several alternative ideas about rusting. When asked about a rusty nail some students think that rust comes from the nail itself, believing that rust is already under the surface of the nail and is exposed during the rusting process. Some students describe rust as a type of mold. Students’ everyday experience with rusting often involves iron getting wet. Consequently, some students are likely to think that rusting happens as a result of the water eating away at the metal, rather than its being an interaction with oxygen in the air. In a survey conducted of English 15-year- old students who were asked about what happens to the mass of a nail after it rusts, one-third said the rusty nail would weigh less, one-third said more, and one-third said it would stay the same. Of these students, just over 10% of those studying chemistry said the mass would increase because the mass of the rust is added to the mass of the nail. There was no indication from their response that the iron in the nail was involved in the formation of rust. Others who understood the reaction explained that the mass would not change because oxygen doesn’t weigh anything (Driver et al. 1994).
Students are likely to think that when something rusts it weighs less because it looks as if parts of the metal are being “eaten away” or the powdery rust is less substantive than the iron (Driver 1985).
Combustion
Some students think that when something is burned in a closed container, it will weigh more because they see the smoke that was produced (AAAS 2009).
Studies of 11- and 12-year-olds’ ideas about the role of air in burning suggest that most know that air is needed for burning, but the function of air is not generally understood. Students may realize that oxygen is necessary for combustion but may not understand how it interacts with the material. Some combustibles are said to have “melted” or “evaporated,” or the combustible substance is thought to be made up of the substances that eventually appear as products (Driver et al. 1994).
Conservation of Mass During Chemical Change
For chemical reactions that evolve gas, mass conservation is more difficult for students to grasp (AAAS 2009).
More than half of a group of 15-year-olds considered to have “above average ability” predicted loss of mass on the combustion of a sample of iron wool. Many students do not recognize the quantitative aspects of a chemical change and the conservation of overall mass (Driver et al. 1994).